In the oil and gas industry, it is important to learn the nature and characteristics of sub-surface formations in order to locate oil and gas deposits. Information gained by drilling a borehole and testing core samples usually is not sufficient to form a decision as to the existence of acceptable levels of hydrocarbons that would justify a drilling operation.
In order to supplement the above bore hole information, induction logging tools such as that disclosed in U.S. Pat. No. 4,455,529 have been used to measure the resistivity or conductivity of sub-surface formations. An AC magnetic field is transmitted by an antenna to induce eddy currents in the sub-surface formations. Variations in the magnitude of the eddy currents (relating to variations in the formation conductivity) are reflected as variations in the received signal. The presence of hydrocarbons in the sub-surface formation is indicated by low conductivity measurements.
An acoustic logging tool acts in a similar manner by transmitting an AC sound wave that propagates through the subsurface formations. Variations in the received signal indicates changes in sound flux that may be caused by formation pockets or cavities in which hydrocarbons are trapped.
Application and power restrictions have prompted the use of higher frequencies (ten to several hundred kilohertz) with shorter pulse widths. The higher frequency operation in the high temperature environment (up to 200 degrees Centigrade) of a logging tool in a borehole gives rise to a deterioration in the received signal which is caused by spurious transmitter distortions such as power spikes, fly-back coupling, Millers capacitance and other noise contaminants. In addition, the switch electronics that is used in the transmitter must be adapted to accommodate high currents of up to several amps that may occur with transient times of less than a hundred nanoseconds in a load or antenna.
Conventional switch electronic systems may include transistor amplifier circuits similar to those disclosed in U.S. Pat. No. 3,912,981 and U.S. Pat. No. 3,921,089. Each is comprised of an input stage which is the source of a signal to be applied through the switch electronics to an inductive load, a biasing current stage comprised of bipolar transistors to supply gate bias voltages to the power end of the switch, and an output stage comprising Field Effect Transistors (FETs) which turn on and off in response to the biasing circuit to supply current to the inductive load.
These traditional designs, however, do not adequately reduce the effects of fly-back coupling, Miller's capacitance, parasitic power spikes, the drain-to-gate and source-to-gate capacitances occurring in the output stage FETs, and the collector-to-base and emitter-to-base capacitive coupling occurring in Bipolar Junction Transistors, to avoid switch time deterioration in a high frequency, high temperature operation that includes currents of 2-3 amps or more with extremely short transient times in a load.